H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

On my most recent flight I noticed a high CHT indication only on the #1 side. When I cut power the CHT dropped. I was about 20 miles from home base so I proceeded at low power and landed with no issue. I did try to bring power back during those 20 miles but each time the CHT rose. When I downloaded the engine data I see that this happened on 6 flights since June when the airplane was new (Bristell with 912ULS) but I didn't notice them at the time. Oddly enough, when CHT#1 rises, CHT#2 falls. At its highest point on the last flight, CHT#1 rose to 259, while #2 dropped to 157. My question is - is that really possible? My understanding is that the Rotax CHT probes are really coolant temp probes. If there is only one coolant system, can one side really be 100 degrees higher than the other, or is it an indicator problem? Any comments would be appreciated.

Whether it is a coolant temp or cylinder temp depends on the vintage of the engine. The heads with coolant temp probes have the probe on top at an angle near the exhaust port. The CHT heads have the probes on the bottom front left and bottom right.

A lot of times a problem like this is electrical and not an actual temp issue. I would check to see if there is a poor or broken ground connection. Another thing you can do is make some jumper wires and switch the leads to the probes and see if the problem switches sides. Also take a look to see if the EGT temps changed during the same time period.